Photomultiplier



Dec. 30, 1958 Filed Ju'Iy 2o, '1956 3 Sheets-Sheet 1 HIS ATTORNEY Dec.30, 1958 A. LALLEMAND 2,866,914

PHOTOMULTIPLIER Filed July 20, 1956 5 Sheets-Sheet 2 FIG.2

INVENTOR. ANDRE LALLEMAND lf2/0%; MW

HIS ATTORNEY Dec. 30, 1958 Filed July 20, 1956 IN V EN TOR.

w ANDRE LALLEMAND BY l HIS ATTORNEY United States had PHOTMULTIPLIERAndr Lallemand, Paris, France, assignor, by mesne assignments, toSchlumberger Well Surveying Corporation, Houston, Tex., a corporation ofTexas This invention relates to electron multiplier and, moreparticularly, to electrostatic secondary-emission photomultipliers.

Photomultipliers are now employed in a wide variety of applications forconverting light energy into signals of useful strength. In anelectrostatic photomultiplier, electrons ejected by radiation falling onthe photocathode are accelerated toward the rst of a seriesr ofv dynodesat successively higher potentials. These dynodes comprising an electronmultiplier have the characteristic of yielding large numbers ofsecondary emission electrons when bombarded with a relatively fewenergetic electrons. By cascading the path of secondary electronsthrough the series of dynodes, a stream of electrons fiowing from thelast dynode to the photomultiplier anode may be derived of a magnitudeproportioned tothe numbers of electrons emitted from the photocathodebut ampliiied by a million or more.

In photomultipliers heretofore devised, this performance may bedisturbed by a variety of causes, several yof which areparticularlysignicant. First, because a perfect Vacuum cannot bemaintained in photomultipliers over a period of time, residual gasesremain which are subject to ionization by collision with acceleratedelectrons. A positive gaseous ion formed by `ionization may be drawn tothe photocathode with suicient energy to eject electrons, thus creatinga spurious signal and leading to unstable operation. This disturbance isfrequently termed ion feedback.

A second disturbance arises when the potential gradient at the surfaceof a dynode becomes so great as to liberate electrons by a process knownas cold emission or field emission. This disturbance vmay ariseparticularly from non-linearities in potential variation along theelectron paths extending between successive dynodes.

A third disturbance arises from the luse of an insulating glass envelopeto enclose the multiplier structure and of insulators to support partsol' this structure. Because of the requirement that successive dynodesbe at diierent potentials, for example, a relatively open space betweensuccessive dynodes is frequently provided through which electrons areguided by focusing arrangements. Such focusing never being perfect inpractice, some 'of the electrons traveling between adjacent dynodesescape to the walls of the glass envelope and eject secondary electronstowardthe anode. A suicient positive charge may thus be developed on aninsulating surface to result in a cold emission discharge and theconsequent generation of spurious noise signals at the photomultiplieroutput terminals. Noise signals may also be derived from so-calledmicrophonics, that is, vibration of the dynode structure in a mannerdistorting the electron path.

vIt isian object ofthe presentV invention to providea new and improvedphotomultiplierin which the above-described disturbances are minimizedor eliminated.

yMore particularly, it is an object of this invention to provide Aaphotomultiplier having an effective arrangement for confining the travelof secondary emission of arent electrons toa path of substantiallyuniform potential gradients while shielding against outsideperturba-tions.

Another object of this-invention is to provide a` photomultiplier ofhigh sensitivity and stabilityv of operation and with a minimum ofresidual gases.

Yet another object is to provide a photomultiplier requiring a minimumnumber of signal carrying leads and hence a minimum number of sealsbetween the photomultiplier `envelope and such leads.

K nStill another object is to provide a photomultiplier of sirrlple,lrugged construction for ease of assembly and reliability of operation. YI

These and other objectsV of the invention are attained by an improvedassembly of photomultiplier Jdynodes arranged not. only to yieldsecondary .emission electrons but to -focus and confine these electronswhile adsorbing positive gaseous ions tending to move toward thephotocathode. .The assemblycomprises spacers enclosingthe i'egionbetweeneach successive pair of dynodes and having low conductivity interiorwalls both to linearize the potential variation between successiveVdynodes and to discharge any particle deviating from the path betweensuccessive dynodes.

Each dynode, except the, last, comprises strips of electron emissivematerial presenting an emissive surface to impinging electrons withpassages between the strips providedfor flow of secondary electronsejected by such impinging electrons. Tofocus the impinging electrons onthe secondary emissive surfaces, each dynode carries a curved wire mesharched over the dynode strips to form a convergent, electrostatic lens.Appropriate portions of these lenses are constructed of a so-calledgetter material, that is, a material adapted to adsorb the ions ofresidual gases contained within the photomultiplier.

TheV invention, together with its objects and advantages will moreperfectly be understood from the following detailed description taken inconjunction with the drawings, in which: g

Fig. 1 is an elevation of the improved photomultiplier of this inventionwith portions including the envelope partially cut away to revealinternal features of construction;

Fig. 2 is an exploded View of the iirst dynode with portions thereof cutaway;

"'Fig. 3 is a sectional view of the photomultiplier of Fig. l takenalong adiametral plane and illustrating only the lower portion of thephotomultiplier, together with a schematic version ofcircuitryforassociation therewith; and

Fig. 4 is a schematic view of the photomultiplier to illustrate itscircuit equivalentitogether with representative paths of chargedparticles.

Throughout the figures like reference numerals are employed ltodesignate similar elements. In accordance with the present invention, aphotomultiplier is shown in Fig. vl comprising an evacuated envelope 10having therein a photocathode 11, an electron multiplier assembly 12 andan anode 13. 1'1' he envelope 10 is conveniently of cylindricalconfiguration and is composed of a material, such as a shock resistantglass, capable of withstanding externally applied pressures and servingas an electrical insulation. Atone end of the photomultiplier theenvelope 10 provides a window 14 transparent to radiation such as lightquanta or photons.

lInteriorly of the window 14 are a barrier layer 15 and avphotosensitive layer 16 of the photocathode l1. The barrier layer 15 istransparent to the. radiations to which the layer r16 -is sensitive.Preferably, the layer 15 is .deposited o n the inner surface of thewindow 14 and is composed of an ,electrically conductive substance asvdescribed in copending application Serial Number 599,207,

3 filed July 20, 1956, by J. P. Causse and A. Lallemand for Photosurfaceand Method of Making Same. In accordance with this application, thelayer further serves chemically to isolate the photosensitive materialof layer 16 from constituents, such as boric oxide, which may be `thephotosensitive layer 16,' eachfhaving the characteristic of emittingelectrons known as photoelectrons upon the incidence of radiationpassing through the transparent window 14 and layer 15. For example, thephotcsensitive layer 16 may be composed of cesium-antimony,sodium-antimony, or any of 4a `variety of intermetallic compounds ofalkali or alkaline earth `metals and antimony, sulfur, selenium,tellurium, bismuth, thallium, or lead.

To raise the signal `represented by the photoelectrons to a usefulstrength, the electron multiplier assembly 12 comprises a series ofdynodes 20a, 20b, 20j and 20k aligned with the photocathode 11 and anode13, the first `dynode 20a being disposed opposite the photocathode 11.As seen in Figs. 1 and 2, the first dynode 20a comprises a centeringring 22a, a plano-convex lens 23a and an axial spacer 24a.

Ring 22a serves to position `the dynode 20a along the 'axis of theenvelope 10. To this end, the ring 22a includes a thin, at annulus 26 ofinsulating material, such `as mica, having a notched circular periphery27 frictiona1- ly engaging the inner wall of the envelope 10, and acircular inner periphery 28 supported in an inwardly crimped portion 29of an annular axially extending band 30. The band is composed of aconductive'material such as aluminum. Below the inwardly crimped portion29 is provided a `downwardly and inwardly directed annular shoulder 31and above it a at upwardly directed "shoulder 32 for engagement with thelens 23a, as hereafter described. The upper shoulder.32 formed in theband extends radially `outwardly therefrom overlying the insulatingannulus 26. The band 30 then provides a conduction path between theouter surface 33 of its axially extending lower portion and'the shoulder32. At the outer periphery 27 of the insulating annulus, uniformly'spaced notches may be provided in order that conductor 35, as well asother support rods (not shown) may extend axially along the inner wallof the envelope 10 to the base 19. i

The plano-convex lens 23a is,like the centering ring 22a, of a unitaryconstruction andis, moreover, arranged for detachable clampingengagement with the centering ring 22a. The lens comprises a rigidannular frame 37 of generally circular cross section flattened in theaxial direction and composed of a conductive material, for ex-` ample,nickel. Formed snugly"about the ring 37 is a channel shaped member 38opening inwardly and in interior pressure contact with the flattenedupper and lower surfaces of the rigid frame 37.` Member 38 is alsoconductive. s

Inserted between the upper `,face of the frameV 37 and the channelmember 38`are vradially extending end portions 39 of tine, arcuate wires`40 lying in spaced axial planes and circularly contoured to define arelatively open, hemispherical mesh having its convex side facing towardthe photocathode 11. ,s

Between the lower face of the rigid ring 37 and the inner surface of thechannel member 38 pressing thereagainst, tabs 42 formed at either end ofdynode strips 43 are inserted to rigidly hold the strips in spacedparallel relation across the aperture defined by the ring 37. The strips43 are bent out of the horizontal plane of the tabs '42 at anangle, `forexample, of 45 to theaxis ofthe Y22a and lens 23a into firm contact.

-larger radius of curvature.

lens in order to present their inclined secondary-emissivc surfaces 44to the photoelectrons from the photocathode 11 for impingement at anangle. For good secondary emission characteristics, the upper surface 44of the strips 43 is, for example, composed of a silver-magnesium alloy.By reason of this inclined strip construction, the dynode 20a may bereferred to as a Venetian blind type.

To minimize ion feedback to the photocathode 11, both the mesh wires 40and the undersurfaces of the dynode strips 43 are composed of a metalhaving the qualities of a getter, that is, the property of dischargingand adsorbing gaseous Amolecules formed when a positively chargedgaseous ion impinges. Satisfactory getter ma terials include titanium,molybdenum, zirconium, tantalum, palladium,`and others.

To the underface 46 of the channel member 38 is secured a wire ring 47sized for snugly fitting within the inwardly crimped portion 29 of thecentering ring 22 when the underface 46 is resting on the shoulder 32.Also attached to the underface 46 are a plurality of spring fingers 48extending downwardly in a generally tangential relation to the annularchannel member 38. When the lens 23a is assembled with the centeringring 22a, the spring fingers 48 will extend under the inwardly crimpedlportion 29 of the centering ring and bear resiliently against theinclined shoulder 31 to cam the centering ring A good electricalconnection is thereby effected, so that the entire lens 23a and the bandportion 30 of the centering ring 22a may be at the same electricalpotential.

The remaining component of the dynode unit 20a, namely, the axial spacer24a, has the form of a cylindrical vband with an interior diameter sizedto snugly receive the band portion 30 of the centering ring 22a. Whenthus frictionally interfitted with the centering ring 22a, the

spacer 24a has its upper surface 49 bearing against the insulatingannulus 26. As seen in Fig. l, the spacer 24a also snugly receives lens23h of the second dynode unit 20b. The bottom edge 50 of the spacer isthen held against the upwardly facing shoulder 32 of the centering ringfor the second dynode unit 2Gb.

In order that the potential gradient between successive dynodes may berendered more uniform, the spacer 24u has a composition which affords alow conductivity path. Since the electrostatic field lies within thespacer and since the inner surface of the spacer makes pressure con tactboth with the centering band 30 and the lens channel member 38, it isonly necessary that the inner surface exhibit conductivity. Hence, thespacer may be composed of an insulating material such as glass, aceramic or the like, preferably with its inner surface having a layer 51of conductive material deposited on it. The layers may, for example, becomposed of gold or silver evaporated upon the inner surface or, ifdesired, a getter material of appreciable conductivity. Moreover, thebulk of the spacer 24 may itself be somewhat conductive. While therequired conductivity of the spacer 24 will depend upon the currentdrawn from the dynodes and the potential difference existing betweenadjacent dynodes, its resistance may be on the order of 200 kilohrnswith about to 200 volts between successive dynodes. Preferably, theresistance will have a low temperature coefficient, especially where thephotomultiplier may be employed in variable high temperatureenvironments such l as are encountered in radioactivity well logging.

While the foregoing description has been particularly directed to thedynode unit 20a, it will apply with equal force to the dynode units 20bthrough 20. A difference will be observed, however, in the later dynodesof the series as .their spacers are shorter in axial length and the wiremesh of the lenses becomes shallower, that is, of a Moreover, the spacer24j is provided withan aperture 52.

The anode 13, preferably formed of a wire grid as seen in Fig. l, isdisposed within a conductive cylinder `53,

known as a Faraday box, having an aperture l54. A `lead-1n `Wire 55passing throughY aseal'56 in t'hebase 19 and through the apertures-52am54-supports the anode 13 firmly. The cylinder -53 may be secured to thedynode 20k conductively.

For efficient production of secondary-emission electrons from the lastdynode 20k, this dynode yis formed as a circular. plate closelyunderlying the grid-like anode'13. The periphery of the dynode-20k Vissized to 'tit within the bottom end of the cylinder S2 and is inelectrical contact` with the inner conductive getter layer 51 of thesame. A conductive lead 57 passing through a seal v58 in the base 19affords electrical contact with the dynode Ztlkand, at the same time,rigidly supports the sameand the cylinder S2 in spaced relation to thedynode 20j.

The rugged mounting of the electron multiplier structure may be observedthen to follow from the snug contact between the insulating annuli 26and the inner wall of the envelope 1t), the snug interfitting of thelens, centering ring', and spacer for the successive dynode units, andthe -rigid supports afforded by theV leads 35, 55 and 57 passing,respectively, out of the base 19 through seals 36, 56 and 58. The lineararrangement of the'Venetian blind dynodes, furthermore, facilitatesready manufacture of the dynode units such that snug intertting maybeobtained with economy.

if desired, additional lead-in conductors (notshown) may be' connectedto each of vthe dynodes20b, 20j in the same manner that conductor 35connects with dynode 20a further lto rigidify` the assembly. As will bemore fully described hereafter, however, 'these additional conductorsare unnecessary to the establishment of suitable potentials 'on thedynodes in view of the employment of theconductive spacersinterconnecting the successive dynodes, but may nonetheless Vbe employedfor this purpose, if desired.

ln Fig. `3 suitable means for energizing-the photomultiplier is showncomprising a D.C. source 60 connected across a potentiometer 61 havingits negative terminal connected to conductor 17 and its positiveterminal connected through resistor 59to the anode 13. Connected to atap 62 in the potentiometer close to its positive terminal isconductor57, whereby the potential on the last dynode 20k is made lesspositive thanf'the anode 13. Coupling means suchas capacitor 63connected to the anode 13 serves to couple the signal'developed acrossthe resistor 59 to an indicating circuit orl thel like (not shown).Conductor 35 connects the. r'st V'dynode 20a with a tap 65' toward thenegative end` of` the'potentiometer' 61. in this manner,'the potential!between the first dynode 20a and the photocathode 11 may be xed at asuitable value governing acceleration of photoelectrons emanating fromthe photocathode. If desired, additional conductors, similar toconductor 35, might be arranged for connecting each of the dynodes 20h,20j appropriately to the potentiometer 61 but such additionalconnections would be largely superfluous in view of the conductivecharacter of the spacers.

In Fig. 4, the spacers 24a, 241 are schematically represented asresistors connecting between the corresponding pairs of dynodes 2da,Zilb, 20j and 20k. This representation illustrates more clearly thesignificance of the spacers in establishing successively higherpotentials proceeding from the photocathode 11 through the first, thenceto the last dynode and nally to the anode' 13. Because the inner wallsof the spacers are conductive, moreover, they tend to rendersubstantially uniform the potential gradient in an aXial direction alongthe walls. The eect of the spacers in conjunction with the planoconve:lenses forming the series of dynodes is exemplied by the representationsof cascaded electron paths 67 stemming from a single photoelectron 68emitted from the cathode 11. Also exemplified are representative paths69 for positive ions 70 formed as collision products of the acceleratedsecondary emission-electrons. These posilli higher potential field ofthe first dynode.

6 tive'ions; indicated in Fig.'4 by a' positivel sign, are shownlmplngingin one iin'stanceupon ahwire 40 of the spherical 'meshlfor'dynode 20e and, in the other instance, impinging `upon the`undersurface of a dynode strip -43 of the -seconddynode"20b. It willfbeappreciated, of course, that only a fragment of the actual electronpaths may be representedfand that the high gain per stage precludes vanintelligible representation of paths through the entire 20a. `Wheneverphotons or other radiant energy impingc' upon the photocathode11,'photoelectrons are ejected and are ldrawn from the region ofthecathode toward the As the photoelectrons approach the convex side ofthe wire mesh 40,

all of themv convergeA inwardly'toward the axis of the photo-tube andvof the first lens.

This results from the convergence of the elec-tric eld `between lensesproduced by the mesh curvature. Hence, the photoelectronsareelectrostatically'attractedtowad the `axis, thereby in- `creasing thecollection efficiency `of the first dynode and preventingphotoelectronsfrom straying to the wall.

vlt/Iost" of the ph'itoele'ctronsv willimpinge upon the secondaryemissive surfaces of strips 43' to eject secondary electrons therefrom.'Since' the secondary electrons may number yin the ratio of'4 to l, `onthe average, relative to the numbers of incident photoelectro'ns,electrons representing-the diiferencebetweenthe captured and the emittedelcctronswill'flow"from lthe-source 60 through the conductor 3S' to thedyndde` 20a. At'the same time, the secondary electrons will' be drawnconvergently toward Ythe second kdynodeZtlb, repeating the-secondaryemission process, and 'the process will thusl cascade through the seriesofdyno'des. `Finally,some^of the secondary electrons emitted by thedynode 20j will pass directly to the anode 13'while perhaps the greaternumber will pass through'the anode 13tstriking vthe last dynode 20k to'produce y'et additional secondary electrons. Substantially 4all of thesecondary electrons eventually result in an electron iow tothe anodez13'yielding a current therethrough substantiallyA linearly proportionedto the intensity `of klight incident upon the photocathode 11. TheWithdrawalof electrons by the secondary emission processtends todrive'each dynode positively, so that electrons flowv from thesourcevthrough conductor 35 land the spacers 'to the dynodes to restore'eachtoits normal potential.

In the event that electrons strike the inner walls of the spacersdespite the convergent elds established by the lenses, the conductivityof the spacers will allow such electrons to be discharged through thelast dynode 20k to the power supply tap 62. Because the conductivity ofthe spacers is sufficiently large to permit an appreciable current topass from the last dynode to the first dynode, however, the very slightcurrent produced by electron collisions with the inner walls of thespacers will have no significant elect upon the proper operation of thephotomultiplier.

Within the lenses, the electrons will generally exhibit cathode 11, thesurfaces which such positive ions likely might strike are prepared forcapturing or adsorbing the ions at the same time neutralizing theircharge. Thus, positive ions may impinge upon the undersurfaces 45 of thedynode strips or upon the wires 40 of the spherical meshes, each ofwhich are composed of getter material.

In this manner, regenerative ion feedback and consequent instabilitiesare avoided. It may be noted that the spherical wre mesh of eachdynode,A being at the potential of the corresponding secondary emissionsurface, enhances both the yield and the collection of secondaryemission electrons.

In the event that the photomultiplier is subject to shock or vibrationalforces applied to the envelope, these forces will be transmitted throughthe centering rings 22a, 22; to the firmly connected lenses and spacersso that the entire assemblage rwill have a unitary motion.

As seen particularly in Fig. 4, the lens structure will produce a higherpotential gradient along the axis of the phototube, but for a shorterdistance, than the potential `gradients in the axial direction`displaced radially from the axis of the tube. Where the expression isused that the spacers tend torender the potential gradient moreuniform,1this should `be understood asrneaning that thepotentialgradient experienced by electrons traversing an axial pathbetween successive lenses or dynodes will experience substantially aconstant potential gradient, although the gradients along adjacent pathsmay be either greater or smaller. i

While a preferred embodiment of the invention has been shown anddescribed, it will `be evident that various modifications may be madewithout departure from the principles of the invention. Thus, in lieu ofa photocathode formed on the interior face of the window 14,

a photocathode might be disposed in front of the iirst dynode at thefocus of a Schmidt type optical system coupled to the source ofradiation. The spherical wire mesh defining the convex surface of theelectrostatic lenses might be formed of wires woven in a crisscrosspattern. In lieu of constructing each of the dynodes with gettermaterial at the undersurface of the wires 40 and at the nndersurfaces 45`of the `dynode strips, onlythe latter tive or so of the dynodes mightbe so constructed, these being in the region where ionization is mostapt to occur.

which may occur to those skilled in the art, the invention is not to belimited except as delned in the appended claims.

8 I claim: 1. In an electron multiplier, a cathode, an anode, a

4plurality of dynodes, each dynode including a circular which confrontsaid anode composed of an adsorptive gettering material, a plurality ofinsulating cylindrical spacers stacked in endwise alinement, a pluralityof cen tering rings extending between the endwise surfaces of Vadjacentspacers and conductively secured to corresponding ones of said frames tosupport said dynodes in spaced axial alinement within the stack of saidspacers intermediate said cathode and anode, envelope means supported inaxial alinement with said stack of spacers by at least two of saidcentering rings for enclosing said cathode vand anode in sealed relationwith said dynodes, a pair of leads extending in sealed relation throughsaid cnvelope means for applying a potential difference between saidcathode and anode, and high-valued resistances ex tending along saidstack of spacers and in electrical contact with said centering rings toprovide a potential divider between said dynodes.

2. A11 electron multiplier, as defined in claim l, where in saidcentering rings are secured in full annular supporting contact withrespect to said dynode frames interiorly of said stack of spacers, eachof said centering rings being secured in full annular supporting contactwith respect to an adjacent one of said spacers.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Rodda: Photo-Electric Multipliers, MacDonald & Co., Ltd.,London, 1953,-pages 53 and 54.

Bruining: Physics and Applications of Secondary Electron Emission,McGraw-Hill Book Co., Inc., New York, 1954, pages 112 and 113.

Zworykin: Photoelectricity and Its Applications, I ohn Wiley & Sons,Inc., New York, 1949, pages 144 and 145.

